Calcium and vitamin D basics: what they do and what the evidence says

Calcium and vitamin D are the two nutrients most often named in advice about bones, and also the two most surrounded by confusion. How much is enough? Do tablets help? Should a blood test be done? This page sets out what each nutrient does, the general intake figures published by named bodies, the upper limits, and where the research is genuinely unsettled. It is general education, and your own doctor has the final word on what suits you. For the wider picture, see our guide to osteoporosis and bone health.

What each one does for bone

Calcium is the main mineral in bone. The bone is also a reserve, and the body can pull calcium from it when the diet runs short. The Bone Health and Osteoporosis Foundation (BHOF) notes that bone can be broken down to release minerals the body lacks, such as calcium when dietary intake is low, which is one reason a long-term shortfall matters. The U.S. National Institutes of Health Office of Dietary Supplements (ODS) states that for adults the main criterion used to set calcium recommendations was the amount needed to promote bone maintenance and neutral calcium balance.

Vitamin D works as a helper. The ODS explains that insufficient vitamin D intake contributes to osteoporosis by reducing calcium absorption. Severe, long-standing deficiency causes rickets in children and osteomalacia (soft, poorly mineralized bone) in adolescents and adults. Vitamin D is also needed for normal muscle development, and the same source notes that very low levels can bring muscle weakness and pain.

Neither nutrient replaces the other. Bone also needs movement, and the effects of loading are covered in exercise and bone loading.

General intake figures from named bodies

The figures below come from the Dietary Reference Intakes set by the Food and Nutrition Board at the U.S. National Academies of Sciences, Engineering, and Medicine, as reproduced by the ODS. Other countries may publish their own values, which can differ. They are population recommendations for healthy people, not personal prescriptions.

For calcium, the Recommended Dietary Allowance is 1,000 mg a day for adults aged 19 to 50 of either sex. From age 51 to 70 it is 1,000 mg for men and 1,200 mg for women, and above age 70 it is 1,200 mg for both. For vitamin D, it is 15 mcg (600 IU) a day for adults aged 19 to 70 and 20 mcg (800 IU) a day above age 70. The U.S. National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) publishes the same figures in its osteoporosis care page.

These amounts count everything you take in, from food, drinks, fortified products and supplements together.

Food versus supplements

Most public guidance starts with food. The ODS lists milk, yogurt and cheese as rich sources of calcium, along with canned sardines and salmon with bones, calcium-fortified soy milk, tofu made with calcium sulfate, and some leafy greens such as kale and bok choy. Its table gives about 415 mg of calcium in an 8-ounce serving of plain low-fat yogurt and about 299 mg in a cup of nonfat milk or of calcium-fortified soy milk. Absorption matters too: the ODS says absorption from dairy products and fortified foods is about 30%, while absorption from some plants may be much lower.

Vitamin D is different, because sunlight on skin also produces it. The ODS notes that sun exposure is one reason blood levels are generally higher than diet alone would predict. People who have limited sun exposure, follow a diet low in vitamin D, or cannot absorb it well from the gut are more likely to be low.

Supplements fill gaps for some people. Whether you are one of them depends on your diet, your health and any medicines, so it is a conversation for your own clinician or dietitian. The next section explains why more is not automatically better.

Upper limits

The Food and Nutrition Board also sets a Tolerable Upper Intake Level, defined as the maximum daily intake unlikely to cause adverse health effects. For calcium, the ODS lists 2,500 mg a day for adults aged 19 to 50 and 2,000 mg a day for ages 51 and over. It adds that higher intakes of supplemental calcium might increase the risk of kidney stones, and that some research has linked calcium supplements with cardiovascular disease, though not all studies agree.

For vitamin D, the ODS lists an upper level of 100 mcg (4,000 IU) a day for adults. It states that vitamin D toxicity is almost always a result of excessive intakes through supplements, and that it can cause high blood calcium and, in extreme cases, kidney failure. Sun exposure alone is not believed to cause toxicity.

Why blood vitamin D testing is a conversation for your own doctor

The usual blood test measures a form called 25-hydroxyvitamin D, or 25(OH)D. Results are reported in either nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL), and 1 ng/mL equals 2.5 nmol/L. That alone causes confusion when a report and an article use different units.

There is a larger problem. The ODS explains that measuring vitamin D status is complicated by the considerable variability of the available assays used by laboratories, so a result can be falsely low or falsely high depending on the assay and the lab. Whether to test at all, how to read a number, and what to do about it depend on your age, health, medicines and symptoms, which is why this is best handled with the clinician who knows your history. Do not change what you take based on a number from a general article.

What the evidence says about supplements and fractures

This is the part that surprises people. Giving calcium and vitamin D supplements to healthy older adults has produced mixed results, and the ODS says so directly. It reports that clinical trials have shown such supplements may raise bone density a little in some postmenopausal women and older men, but it is not clear whether they reduce falls and fracture rates.

The ODS reviews several analyses that point in different directions. One meta-analysis of eight trials in 30,970 adults older than 50 found that calcium plus vitamin D reduced total fractures by 15% and hip fractures by 30%. Another review, which pooled many trials, found calcium alone had no effect on hip fracture risk, and a combined calcium and vitamin D analysis of six trials in 49,282 older adults found smaller reductions, 6% for any fracture and 16% for hip fracture.

The U.S. Preventive Services Task Force, as summarized by the ODS, looked at 11 trials in 51,419 community-dwelling adults aged 50 and older without osteoporosis, vitamin D deficiency or earlier fractures. It concluded the evidence was insufficient to judge the balance of benefits and harms of supplementation for preventing fractures, and it recommended against 400 IU or less of vitamin D and 1,000 mg or less of calcium for that purpose. On falls, it concluded with moderate certainty that vitamin D supplementation does not reduce falls in adults 65 and older without osteoporosis or deficiency. Those findings apply to the groups studied, and people with a diagnosed deficiency or osteoporosis were not the focus.

The honest summary is that getting enough through the diet is widely endorsed, while the benefit of extra tablets for fracture prevention in people who are not deficient remains uncertain. If you are already being treated for osteoporosis, how calcium and vitamin D fit with your medicines is covered in the medicines overview, and the final decisions rest with your doctor. Preventing falls matters on its own terms, and fall prevention at home covers it.

The short version

Calcium supplies the mineral for bone and vitamin D helps the body absorb it. Named bodies publish general daily intakes and upper limits, and meeting them through food is the usual starting point. The evidence for supplements preventing fractures in people who are not deficient is uncertain, and a blood vitamin D test is best discussed with your own doctor.